I. Field of the Invention
The present invention relates to computer controlled fiber placement machines which apply bands of fiber tows to a mandrel or the like to construct components made up of the fiber tow material. More particularly, the present invention relates to controlling the tension on the fiber tows in the active application mode of operation of such machines as well as to control tow take-back to avoid snap-back after rethreading a tow.
II. Description of Prior Art
By way of background, a computer controlled fiber placement machine includes a creel assembly which feeds a band of individual fiber tows under tension to a tool such as a fiber placement head. Each fiber tow is typically provided from a respective spool mounted for rotation on a motor driven chuck on the creel assembly. Each tow is individually threaded from the chuck-mounted spool and through a guidance structure such as over various rollers and on to the fiber placement head. After each tow is threaded, any slack in the tow must be taken up to place the tow under tension for proper application by the fiber placement head.
The fiber placement head is mounted to a robot wrist, for example, and is thus movable under program control through a wide variety of spatial orientations. As the fiber placement head moves about, one or more of the fiber tows are to be applied to either a stationary or rotatable mandrel, for example, to construct a component such as an air foil. Where the mandrel is rotatable, it too may be under program control. During application of the fiber tows to the mandrel, it is desired to maintain the tows under tension between say 1 to 10 lbs. However, the desired or necessary tension may vary as a function of speed of application such as caused by rotation of the mandrel and/or movement of the tow by the fiber placement head, for example. To this end, it has been proposed to provide a computer driven servo-control tensioner by which to control tension under program control as the tows are being applied, i.e., in the active application mode of the machine.
More specifically, it has been proposed to provide a closed-loop microprocessor based servo-controlled tensioner which accepts programmed tension setpoints from a computer numerical control (CNC) controlling the overall machine, and to vary the signals to the motor driven chuck in an effort to achieve and maintain the commanded tension setpoint. As will be appreciated, users of computer controlled fiber placement machines typically program the CNC in so-called world coordinates which define selected points of tow application relative the frame of the machine, such as at the centerpoint of the robot wrist supporting the fiber placement head. The user may also pre-program tension setpoints at these selected points or world coordinates. The CNC manipulates the world coordinate inputs to divide them into a plurality of incremental tool paths between each pair of world coordinates input by the user so as to define a plurality of incremental moves of the various parts of the machine by which to cause the fiber tows to be applied between the world coordinate pairs. At each new world coordinate, the CNC provides the programmed tension setpoint (if different from the prior tension setpoint) to the tensioner microprocessor to define the tension which is to be maintained for each incremental move of the machine parts to the next world coordinate. To this end, the tension setpoints from the CNC may be utilized by the tensioner microprocessor in a feedback loop which compares actual tension on the tow and the commanded tension setpoint to generate a tension error signal. Typically, the feedback loop also includes a filter to compensate for known characteristics of the tensioner such that the tension error signal is modified in a predetermined manner such as by proportional, integral and/or derivative functions as is well known. The filtered tension error signal is coupled to the chuck motor and causes the motor to tend to rotate in a direction which would either reel-off or spool-up the tow so as to decrease or increase tow tension, respectively, in an effort to achieve and maintain the commanded tension.
Experience has shown, however, that undesirable fluctuations in actual tension may still be encountered with such a closed-loop servo-control system. For example, the tensioner system must be able to maintain a tension setpoint as low as 1/2 pound on the tow even while the tow experiences acceleration or other disturbances on the tow. Because low tension setpoints may thus be involved, the allowable tension error is necessarily extremely small such as less than 1/4 pound. However, a variety of disturbances have been encountered in operating the tensioner system which can cause large dips and spikes in the tension making it difficult to maintain tension control. Such dips and spikes may occur not only from acceleration of the tow but also from mechanisms used in the machine which strike the tow or when the tow snaps-off of the spool such as tends to occur with tacky fiber tows or tow pregs as the tow pays off of the spool. On the other hand, the tensioner system must also be able to maintain much larger tensions on the tow, for example, upwards of 15 pounds. Yet, tension control is to be accomplished independent of the type of composite tow material and size or weight of the spool or the material. Accordingly, improvements to closed loop tension servo-controls are believed necessary.
Additionally, whenever a new spool is to be placed on the creel assembly, or a fiber tow breaks, it may be necessary to thread the tow from the chuck-mounted spool, through the rollers of the guidance structure, and into the fiber placement head. This threading process is normally accomplished with the tensioner disabled and by an operator manually pulling off lengths of tow from the spool and threading the loose tow through and around the appropriate rollers. As the operator pulls off lengths of tow, however, there may be a tendency for too much of the tow to reel-off. After the tow is threaded, control of the tensioner is to be restored to the CNC whereupon tow take-back may occur to tighten up the tow and place it under tension. As the CNC program is predefined without knowledge of the amount of reel-off, upon restoring control to the CNC, there may be a sudden and undesirable snapping action on the loose tow as the active servo-controlled drive seeks to restore the tension by respooling the tow. Accordingly, improvements to the tensioner are desired to reduce snap-back as the active application mode is restored and on tow take-back.